Using self types for trait composition

While using traits in a small Scala project, I finally understood when to use self types. Self types are often characterized by Scala newbies as an incomprehensible language feature with no obvious usage. After coding with traits for a while I understood that self types are an essential information hiding tool for composing traits.

I am coding a small JMX library in Scala, similar to Spring’s JMX module and to the jmxutils library. In my project I have a MBeanInfoAssembler trait, that is a strategy interface, whose role is to create a ModelMBeanInfo from a Class. Implementations of that trait will use reflection to discover the attributes and operations that will be used to export instances of the Class to JMX.

In the above code I have removed the implementations of the attributes and operations methods because they are not important for this example. What is important is the createMBeanInfo method which just calls the ModelMBeanInfoSupport constructor with the results of the attributes and operations methods. If we leave the code like this, when we try to provide a second implementation of the MBeanInfoAssembler trait we will probably copy-paste that method’s code.

In Scala, traits can have concrete methods so our first though is to move the implementation of the createMBeanInfo method to the MBeanInfoAssembler trait. After doing that our trait becomes:

This refactoring has the obvious advantage of removing the repetition that would be needed to create multiple implementations of MBeanInfoAssembler but also one major disadvantage over our previous code.

In order to support the implementation of the createMBeanInfo method the MBeanInfoAssembler trait gets updated with two new public abstract methods (attributes and operations). Those methods are useless to the clients of the trait, who only want to create a ModelMBeanInfo from a Class, and are there only to support the trait’s implementation. To correct this defect in our design we need to do another refactoring by introducing a new trait and connecting the two traits together using a self type annotation.

If you are new to Scala you might wonder what is a self type? In the Scala documentation self types are described as a way to “declare the type of the value this explicitly” which might be accurate but sounds at least confusing for a newbie like me.

In a well designed object-oriented system we achive reuse and flexibility when our system has lots of small objects that have a single responsibility and are explicitely connected using interfaces that represent the role of the connection. Usually those connections are made using constructor arguments. The power of such system comes from the testability and reusability of the small objects and the flexibility of composing those objects to solve a particular problem.

Since traits in Scala can have code (method implementations), if we want to create an equally well designed and flexible system using traits we need to be able to do the same. The problem is that traits do not have constructors so we need a new construct to declare trait dependencies and also a way to compose traits together. The solution to the first problem are the self type annotations and the solution to the second is multiple inheritance.

In the previous example the MBeanInfoAssembler trait had many responsibilities assigned to it. It must be able to create a ModelMBeanInfo given a Class and it must also extract any metadata in order to find the right attributes and operations for the given Class. We decide to move the second responsibility to a new trait called MBeanMetadataExtractor that has the attributes and operations methods.

Now we need to update the MBeanInfoAssembler trait and to somehow connect it to the MBeanMetadataExtractor trait so it can use the attributes and operations methods. To do this we will use a self type annotation.

The syntax of the self type annotation is this: MBeanMetadataExtractor => and it tells the compiler that a MBeanInfoAssembler can be implemented in a class only if that class also implements the MBeanMetadataExtractor trait. Using this assumption the compiler makes the methods of the MBeanMetadataExtractor available for use in the MBeanInfoAssembler (in our case the attributes and operations methods).

The class SimpleMBeanInfoAssembler now has to implement both MBeanInfoAssembler and MBeanMetadataExtractor.

By doing the above changes we were able to hide the attributes and operations methods from the clients of the MBeanInfoAssembler trait. Obviously those methods exist in the public interface of SimpleMBeanInfoAssembler but in a well designed system other classes will connect with SimpleMBeanInfoAssembler by using one of its traits (interfaces) so this is not a problem.

In conclusion, if we want to move method implementations inside traits then we risk polluting the interface of those traits with abstract methods that support the implementation of the concrete methods and are unrelated with the main responsibility of the trait. A solution to this problem is to move those abstract methods in other traits and compose the traits together using self type annotations and multiple inheritance.